Is it permissible to give zakat to a debtor threatened with imprisonment, knowing that its time has not yet come, and is he considered among those in debt (al-gharimin)? What about his debts that have not yet been collected?

The category of debtors (al-gharimin) is one of the eight categories for the distribution of zakat. If your sister's son is able to collect his debts and pay off what he owes, then it is not permissible to give him from zakat because he is not a debtor (gharim). However, if he is unable to collect all or some of his debts, then he should be given from zakat an amount that enables him to pay off his debt. There is no harm in expediting the zakat for this purpose.